USGS-OFR-92-528 USGS-OFR-92-528 U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY A geologic overview and one-day field guide of the Taos Plateau volcanic field, Taos County, New Mexico Ren A. Thompson Nancy J. Open-File Report 92-528 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or stratigraphic nomenclature. 1992 1. USGS, Denver, Colorado. 2. New Mexico State University, Las Cruces, New Mexico. INTRODUCTION The Rio Grande rift region of northern New Mexico provides an excellent opportunity to examine the interplay of volcanism, extensional tectonism, and sedimentation within the only active continental rift in North America. This field guide provides researchers and students with an overview of the volcanology, petrology, and geochemistry of the Miocene to Pliestocene volcanic rocks of the Taos Plateau and their relation to rift structure and tectonics. Structure of the Rift Basins of Northern New Mexico The Rio Grande rift is a late Cenozoic feature in continental crust of Proterozoic age (1.7-1.4 Ga) which, prior to rifting, experienced deformation during the Uncompaghre (late Paleozoic) and Laramide (late Cretaceous to Eocene) orogenies. Rift basins exhibit the same N-S trend as Laramide compressional structures in the southern Rocky Mountains, and are located near the eastern margin of the region affected by Laramide deformation. The rift is a major physiographic and structural feature of the southern Rocky Mountains extending northward from a poorly defined terminus in northern Chihuahua, Mexico, through New Mexico, and into central Colorado (Tweto, 1979). Normal faulting, regional uplift, high heat flow (Decker and others, 1984), and scattered occurrences of volcanic rocks of late Cenozoic age associated with rifting are present as far north as southern Wyoming (e.g., Leucite Hills). The Rio Grande rift comprises two segments, the northern segment extending northward from Socorro, New Mexico, into Colorado and the southern segment extending south from Socorro into Mexico (Cordell, 1978). In the regionally broader southern segment, the tectonic style and physiography resemble that of the Basin and Range province: extension is distributed over several wide basins, lithospheric thinning and regional extension have reached an advanced stage, and regional elevations are lower than in the north. In the northern segment, rift basins separate the Colorado Plateau on the west from the High Plains to the east and are confined to a narrow axial zone superimposed on a region of high elevation. During the early stage of rifting, extension-related faults tended to have northwest trends (Aldrich and others, 1986), whereas later Pliocene to Quaternary extension in the Rio Grande rift has produced normal faulting within the axial zone of the rift with predominantly N-S orientations. The younger phase of rifting which followed a period of relative tectonic and magmatic quiescence in the late Miocene (20-12 Ma), is characterized by a narrowing of rift basins, where early basin-fill sediments of the Santa Fe Group are commonly found on the uplifted blocks adjacent to present rift basins (Golombek, 1983). The post-Miocene rejuvenation of rifting was accompanied by widespread, dominantly basaltic volcanism, active regional uplift, and large differential movements on basin-bounding faults that created deep asymmetric basins. In the area of the northern rift from Albuquerque, New Mexico, to Leadville, Colorado, the rift basins are arrayed in en echelon patterns and are offset along commonly northeast-trending oblique structures (fig. 1). The intense regional northeast structural grain of Proterozoic basement rocks control northeast-trending Cenozoic structures. For example, both the graben faults of the resurgent dome of the Valles caldera (Jemez volcanic field) and the Embudo fault zone (the southern structural boundary of the San Luis basin) strike to the northeast (fig. 1). The Embudo fault zone in addition to several other northeast- trending faults serve as boundaries, or accommodation zones, that separate differentially tilted, rhomb-shaped structural blocks with longest dimensions that are generally coincident with the axis of the northern rift. Independent tilting and rotating of these blocks as structural units bounded by reactivated zones of weakness, produced the pattern of en echelon offsets and opposing tilts of the basins and uplifts of the northern rift segment. SANGRE DECRISTO MOUNTAINS RATON-CLAYTON VOLCANIC FIELD LATIR OLCANIC FIELD TIMBER MTN. SAN JUAN BASIN AND BRUSHY MTN. OCATE VOLCANIC FIELD EXPLANATION MIOCENE AND YOUNGER VOLCANIC ROCKS SEDIMENTARY ROCKS IN THE RIO GRANDE RIFT (Miocene to Holocene) MIDDLE TERTIARY VOLCANIC ROCKS INTRUSIVE ROCKS (Laramide to lower Miocene) PALEOZOIC AND MESOZOIC SEDI­ MENTARY ROCKS PRECAMBRIAN ROCKS NORMAL FAULT (dashed where inferred; bar and ball on downthrown side) C ALDERA BOUNDARY TREND OF JEMEZ ZONE Figure 1.-Generalized geologic map of the Southern Rocky Mountains, showing location of the middle to late Tertiary volcanic fields of the northern Rio Grande rift region and their proximity to major Precambrain uplifts and the Jemez zone. The San Luis basin segment of the Rio Grande rift includes the San Luis Valley and the Taos Plateau underlain by lavas of the Taos Plateau volcanic field. The San Luis basin is broken into two east-tilted blocks by a northeast-trending fault zone (Sr. side up) that forms the northern boundary of the structurally high San Luis Hills and creates the reentrant of the front of the Sangre de Cristo Mountains in southern Colorado (fig. 1). Patterns of gravity anomalies of the central San Luis basin show the boundary between these blocks (Keller and others, 1984). The two San Luis basin sub-blocks have a weak component of southerly tilt as well as a more dominant easterly tilt. This southeasterly dip within the basin opposes the pronounced northerly tilt of the adjacent Sangre de Cristo block (Lipman, 1981), emphasizing the independent motion that characterizes these blocks. On the south of the Embudo fault zone, the Espaftola basin block has apparently undergone counterclockwise rotation (Brown and Golombek, 1985), resulting in compressional structures at the northeastern end of the Embudo zone near Taos and in extension at the southwestern end at Velarde. The northeast-trending Jemez zone is defined most clearly by an alignment of Miocene and younger volcanic fields (fig. 1) extending from the southeastern margin of the Colorado Plateau (Mt. Taylor volcanic field) through the axis of the rift (Jemez volcanic field) to southeastern Colorado-northeastern New Mexico (Raton- Clayton volcanic field). Thus defined, the Jemez zone crosses several physiographic provinces. The occurrence of these volcanic fields along the Jemez zone has been interpreted as an indication that a major Precambrian crustal shear or suture zone, reactivated by extensional stresses, has acted as a conduit for the mantle-derived magmas that regionally dominate post-Miocene volcanism (Baldridge and others, 1984). However, the striking alignment of the volcanic fields is not matched by evidence that shallow structures within the northeast Jemez zone are important in controlling vent alignments within each volcanic field. Taos Plateau Volcanic Field The Taos Plateau volcanic field, in the San Luis basin of northern New Mexico, is one of the largest and most diverse volcanic fields associated with the Rio Grande rift and the Jemez zone, covering approximately 1500 km2. Compositionally diverse volcanic rocks were erupted over a period of 24 million years (from 26 Ma to 2 Ma) and include tholeiitic basalts, trachybasalts, basaltic trachyandesites, calc-alkaline andesites, dacites, and rhyolites. Volcanism of three distinct ages is recorded in the Taos Plateau volcanic field (table 1). Early-rift (26-22 Ma) calc-alkaline lavas and pyroclastic rocks associated with the Latir volcanic field are preserved as intra-rift horsts. No volcanic activity has been dated on the Taos Plateau between 20 Ma and 10 Ma, corresponding to a postulated rift-wide mid-Miocene tectonic and magmatic lull (20-12 Ma). However, sporadic basaltic volcanism occurred in the northern Rio Grande rift on the flanks of the Taos Plateau, to the west in the Tusas Mountains, and to the east in the Sangre de Cristo Mountains. A single eruption of quartz latite, Cerro Chiflo, has been dated at 10.2 Ma and represents the only exposed remnant of volcanic activity in the Taos Plateau volcanic field between the early-rift and voluminous Plio-Pleistocene volcanism. Olivine tholeiites, calc-alkaline olivine andesites and two-pyroxene dacites, and rhyolites erupted from 5 Ma to 2 Ma and constitute the predominant volcanic cover of the Taos Plateau. Oligocene to Miocene volcanic rocks Dacite, andesite, and rhyolite lavas, and rhyolite ash-flow tuffs exposed on an intrarift horst at Timber Mountain and Brushy Mountain in the San Luis basin (fig. 2) are clearly associated with volcanic rocks of the Latir volcanic field (Thompson and others, 1986). All units exposed on the horst post-date the caldera-forming outflow sheet from the Qjiesta system (fig. 3; the Amalia Tuff) and represent a regeneration of metaluminous magmas similar to the Questa plutons. Lipman (1984) presents a model for the Questa system in which precaldera calc-alkaline magmas, represented by the intrusions, evolve toward more silicic and alkalic compositions, resulting in the mildly alkalic Amalia tuff. Post-caldera magmas return to metaluminous compositions. Ceiro Chiflo Brushy Mtn. -36°40' Umber Mtn. Taos Plateau 36°30f -36*20' Pilar Figure 2.-Generalized location map depicting field guide stop locations and some of the prominant physiographic features discussed in the text. Two sequences of volcanic rocks are exposed on the horst blocks (Thompson and others, 1986). The lower sequence is dominated by dacite (plag + cpx + opx + Fe-Ti oxides ± hbld), but includes basaltic andesite (ol + cpx + plag) and low-silica rhyolite (plag + san + qtz + bt + Fe-Ti oxides). Dacites contain quenched andesitic micropillows, high-silica rhyolite glass, and sanidine and oligoclase xenocrysts.